This invention concerns thin layer resistive heaters of the type often used in conjunction with solid plate surface heaters in which a thin resistive metal layer is mounted on the underside of a nonconductive glass ceramic plate, such as currently incorporated in some designs of cooking ranges. Such thin layer heaters have in the past taken two basically differing forms, i.e., film heaters in which the resistive layer is bonded to the underside of the glass ceramic plate, or etched foil heaters in which a thin layer of a suitable resistive metal is formed into a pattern by an etching process and is mounted or secured beneath the glass ceramic plate.
An example of a film-type heater is disclosed in U.S. Pat. No. 3,883,719, while an etched foil-type heater is disclosed in U.S. Pat. No. 4,032,750, both of which are assigned to the assignee of the present application.
Commonly associated with such heaters is a temperature sensor which is positioned adjacent the heating element such as to sense when the element temperature reaches an unsafe level. The sensor is associated with a safety circuit to discontinue or interrupt the power to the heater in the event the heater reaches a temperature which endangers the heater element or which could damage the glass ceramic plate.
Such sensors are generally discrete components such as a length of insulated Nichrome wire, mounted directly beneath the heating element. This approach adds significantly to the cost of the assembly since the element must be separately assembled into the heater. In addition, the cost of the material for the temperature sensor itself is added to the cost of manufacture of the unit. Such temperature sensors generally must be insulated electrically from the heating layers in the case of the film heaters such as to not produce a short circuiting between turns in the pattern.
Typically, such thin layer heaters comprise a pattern in which a central opening is formed in order to produce a proper temperature distribution across the area of the heating layer. This temperature sensor has sometimes been located in the region of this central opening.
It is the object of the present invention to provide a temperature sensor for such thin layer heaters which does not require a separate discrete temperature sensor and which does not involve additional assembly labor in installing such temperature sensors and which is compatible with either film or etched foil-type heaters.